The present invention relates to a flexible pipe for transporting, over long distances, a fluid which is under pressure and possibly at a high temperature, such as a gas, petroleum, water or other fluids. The invention relates most particularly to a pipe intended for offshore oil exploration. It relates especially, first, to the flow lines, that is to say flexible pipes unwound from a barge in order to be laid generally on the bottom of the sea and connected to the subsea installations, such pipes working mainly in static mode, and, secondly, to the xe2x80x9crisersxe2x80x9d, that is to say flexible pipes which are unwound from a surface installation such as a platform and are connected to the subsea installations and most of which do not lie below the seabed, such pipes working essentially in dynamic mode.
The flexible pipes used offshore must be able to resist high internal pressures and/or external pressures and also withstand longitudinal bending or twisting without the risk of being ruptured.
They have various configurations depending on their precise use but in general they satisfy the constructional criteria defined in particular in the standards API 17 B and API 17 J drawn up by the American Petroleum Institute under the title xe2x80x9cRecommended Practice for Flexible Pipexe2x80x9d. Reference may also be made to documents FR 2 654 795 A, WO 98/25 063 A and FR 2 744 511 A.
A flexible pipe generally comprises, from the inside outward:
an internal sealing sheath made of a plastic, generally a polymer, able to resist to a greater or lesser extent the chemical action of the fluid to be transported;
a pressure vault resistant mainly to the pressure developed by the fluid in the sealing sheath and consisting of the winding of one or more interlocked profiled metal wires (which may or may not be self-interlockable) wound in a helix with a short pitch around the internal sheath;
at least one ply (and generally at least two crossed plies) of tensile armor layers whose lay angle measured along the longitudinal axis of the pipe is less than 55xc2x0 C.; and
an external protective sealing sheath made of a polymer.
Such a structure is that of a pipe with a so-called smooth bore. In a pipe with a so-called rough bore, a carcass consisting of an interlocked metal strip, which serves to prevent the pipe being crushed under external pressure, is also provided inside the internal sealing sheath.
The pressure vault consists of a winding of non-touching turns so as to give the pipe a degree of flexibility. The expression xe2x80x9cnon-touching turnsxe2x80x9d is understood to mean turns between which a certain space or interstice, called hereafter xe2x80x9cgapxe2x80x9d, is left, which gap may be greater the larger the wound profiled wire.
Due to the effect of the internal pressure and/or of the temperature developed by the fluid, the internal sealing sheath, which is relatively soft, is pressed against the internal face of the pressure vault and has a tendency to penetrate one or more inter-turn gaps. The penetration or creep tendency of the sealing sheath increases with the duration and/or the severity of the operating conditions of the flexible pipe, for example when the fluid to be conveyed flows under a high pressure (several hundred bar) and/or at a high temperature (greater than 100xc2x0 C.), a high temperature generally reducing the rigidity of the internal sealing sheath. When the sealing sheath gradually penetrates the gaps, either cracks are produced, which thus affect the sealing function of the internal sheath, or even one or more local fractures of the internal sheath occur, the consequence of such incidents being an intrusion of the fluid to the outside of the internal sheath, which no longer fulfils the required sealing.
Several solutions have been proposed for limiting or trying to prevent the creep of the internal sheath into the gaps between the turns of the pressure vault, such as the choice of a thicker and/or stronger material for the sheath, but this generally results in an increase in the manufacturing cost of the pipe and often complicates its manufacture. A very advantageous solution has been proposed in document FR 2 744 511 A, which consists in winding an anti-creep tape having specific characteristics around the sealing sheath, this tape forming a sublayer lying beneath the internal face of the pressure vault, but the effectiveness of this solution encounters limitations at high pressures and for high flexible pipe diameters.
The problem associated with inter-gap creep is aggravated when the pipes used are at great depth. This is because the profiled wires used for the pressure vaults of the pipes intended for shallow and moderate depths and for small or moderate flexible pipe diameters (typically less than 12 inches, i.e. 30 cm) are of relatively small cross section and they form, after winding and interlocking, a relatively compact annular volume between its internal face and its external face, despite the presence of gaps which remain small (typically about 1 mm to 5 mm). In the case of pipes for great depths and/or of large diameter (typically greater than 30 cm), it is necessary to use a profiled wire of larger cross section having a larger moment of inertia. The gaps then increase (possibly reaching up to about 10 mm) and the solution recommended in document FR 2 744 511 A is no longer valid. To limit the weight of such a pipe for great depths, it would be desirable to be able to use sections with an H-shaped or I-shaped cross section, the moment of inertia/weight ratio of which is markedly more advantageous than for sections typically having a T-, U- or Z-shaped cross section and for variants thereof. These sections have already been proposed in the specific context of flexodrilling, for example in document FR 2 229 913 A. Such wires create, after winding, an annular vault volume which is less compact than that obtained with wires of T-, U- or Z-shaped cross section and of the same diameter; the vault is lightened, since between its internal and external faces it has a substantially confined helicoidal empty volume, bounded by the facing flanges of the wound wire and its web wound approximately radially. However, with wires of large cross section and having a large gap between non-touching turns, the presence of this volume just to the rear of the internal gap aggravates the risk of creep significantly since this volume constitutes an almost unlimited exit for the material which creeps via a gap. Within the context of flexodrilling and for a different purpose (to seal the vault), it has been proposed to fill this volume with a light elastic material, such as rubber, but this solution makes the pipe excessively rigid; this is because the rubber, which is volumetrically incompressible and confined in said helical volume of the pressure vault, is substantially undeformable and counters the flexibility of the pipe.
The problem of the creep of the internal sealing sheath of a flexible pipe suitable for great depths or for large diameters is therefore unsolved at the present time and the objective of the invention is specifically to solve it.
The present invention achieves its objective by virtue of a flexible tubular pipe of the rough-bore or smooth-bore type, that is to say comprising at least, from the inside outward, an internal sealing sheath made of a polymer material, a cylindrical pressure vault having an external face and an internal face placed over the internal sheath, the vault consisting of the winding of an interlocked metal profiled wire wound in a helix with a short pitch and with a gap between the turns, at least one ply of tensile armor layers wound with a long pitch, and an external protective sealing sheath made of a polymer, characterized in that the vault includes an elongate overlay element at least partially masking the gaps of the profiled wire facing the internal sheath. The elongate, preferably flat, overlay element is advantageously placed helically in and to the rear of the internal face of the pressure vault so as not to be projecting with respect to the annular volume of the vault. A radial confinement in terms of thickness of the gaps is therefore produced and the risk of creep of certain regions of the internal sheath into the gaps of the metal vault is minimized. It may be seen that, unlike the known anti-creep tape, which constitutes an additional layer intermediate between the sealing sheath and the pressure vault, the overlay element of the present invention is a constituent part of the vault itself and is integrated therewith, even if it may be produced in a separate form; this is because, depending on the two main embodiments of the invention, the overlay element of the invention is produced by an attached wire which is inserted into the internal face of the vault or by parts of the profiled wire itself by an overlap on the internal face of the vault.
The invention is applicable to all kinds of wire cross sections, which may or may not be self-interlockable (and in the latter case, they are interlocked by an interlocking wire). However, the profiled wire of the pressure vault advantageously has a lightened cross section in the form of an H on its side or an upright I; the web of the section is wound approximately radially and its flanges define the confined volume, access to which is prohibited by the overlay element which bars the gap on the internal face of the vault.
The invention will be more clearly understood with the aid of the description which follows, with reference to the appended schematic drawings showing, as examples, embodiments of the flexible pipe according to the invention. Further advantages and features will become apparent on reading the description.